The present invention relates to a skew compensation circuit and more particularly to a skew compensation circuit which can compensate for changes in skew according to changes in external environments and processes.
Generally, transistors in a semiconductor device do not correspond to the designed threshold voltage, oxide thickness, gate, ISO and active resistance or the like, due to changes in external environments and processes which results in signal skew. Additionally, an operation for verifying changes in external environments and processes is very difficult.
In order to simulate and verify changes in external environments and processes, phased conditions such as ‘FF, FT, FS, TF, TT, TS, SF, ST, SS’ are used. Based on the ‘TT,’ there are predetermined skew differences between the respective conditions. Herein, ‘F’ means a fast condition, ‘T’ means a typical condition, and ‘S’ means a slow condition.
For example, as shown in FIG. 1, a conventional signal generating circuit generates an output signal OUT using an inverter chain 10, a NAND gate NA1, and inverters INV1, INV2. In this example, the slower the condition, the larger delay of the signal becomes due to the inverter chain 10.
In other words, in the typical condition as shown in FIG. 2A, an input signal IN1 is delayed by a predetermined amount by the inverter chain 10 while an input signal IN2 is inverted by the inverter INV1. The NAND gate NA1 NAND-combines a delayed signal IN1_DLY outputted by the inverter chain 10 and an inverted signal IN2B outputted by the inverter INV1. The inverter INV2 receives an output of the NAND gate NA1 and outputs a normal output signal OUT.
However, in the slow condition as shown in FIG. 2B, the inverter chain 10 may vary the delay amount according to changes in the transistors of the inverter chain 10, such as changes in a threshold voltage, oxide thickness, gate, ISO and active resistance or the like. Therefore, a timing margin between the delayed signal IN1_DLY and the inverted signal IN2B may be insufficient to output a normal signal OUT from the inverter INV2 or may not be sufficient to output a signal at all.
Circuit malfunctions due to a change in skew are not a big problem if changes in the external environment and processes can be accurately predicted. However, if the changes cannot be accurately predicted, it is difficult to correct the circuit malfunctions.
Further, although a certain degree of skew change can be accurately judged according to the prediction of changes in the external environment and processes, when the change in skew is severe, a further process to add or revise the circuit in order to compensate for the change in skew is needed. This extra process requires more time and added costs.